Microelectromechanical systems (MEMS) variable optical attenuator

ABSTRACT

Disclosed is an MEMS variable optical attenuator comprising a substrate having a planar surface, a micro-electric actuator arranged on the planar surface of the substrate, a pair of optical waveguides having a receiving end and a transmitting end, respectively, and coaxially aligned with the other while being arranged on the planar surface, an optical shutter movable to a predetermined position between the receiving end and the transmitting end of the optical waveguides, and driven to move by the micro-electric actuator, and a surface layer formed on the optical shutter, having reflectivity less than 80% so as for incident light beams to partially transmit thereinto, and having a characteristic of light extinction, thereby extinguishing the partially transmitted light beams therein.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical attenuator that usesan element of a micro-electro-mechanical system (MEMS) device, and moreparticularly to an MEMS variable optical attenuator having an improvedoptical shutter for regulating optical power of an optical signal bypartially intercepting incident light beams.

[0003] 2. Description of the Related Art

[0004] An optical attenuator for use in optical telecommunicationnetworks is an optical component for delivering beams of light passingout an exit end of an optical waveguide to an incident end of an opticalwaveguide by causing insertion loss to the light beams.

[0005] Generally, optical power levels are regulated over wide rangesbased on a configuration of an optical telecommunication system. Forexample, the optical power levels are determined by an opticaltransmission loss typically varied based on a length of an opticaltransmission line, the number of connection points of optical fibers,and the number and performance of optical components such as opticalcouplers coupled to the optical transmission line. An optical attenuatoris needed in optical telecommunication networks to reduce an opticalpower when an optical signal with an excessive power level greater thanan allowed power level is received to an optical signal receiver. Theoptical attenuator further may be used in evaluating, adjusting andcorrecting telecommunication equipments and optical measurementequipments.

[0006] Such optical attenuators are classified into two types, a fixedoptical attenuator for reducing an optical power by a fixed amount ofattenuation and a variable optical attenuator (VOA) capable ofattenuating an optical power by a varied amount of attenuation based onuser's requirements. The optical attenuator is required to be producedat low cost with high reliability and small size.

[0007] To satisfy such requirements, an optical attenuator that uses anelement of an MEMS device has been suggested. Such MEMS opticalattenuator is realized by forming a microstructure acting as an actuatoron a substrate such as silicon using thin film processing technology.Generally, an MEMS actuator is driven to move by a driving force causedby thermal expansion or an electrostatic force. As the MEMS actuatormoves, an optical shutter coupled to the MEMS actuator is displaced soas to be inserted into a gap between two optical waveguides, therebypartially intercepting light beams traveling from a transmitting end (orthe exit end) of the optical waveguide such as an optical fiber to areceiving end (or the incident end) of the optical waveguide.

[0008]FIG. 1 illustrates a perspective view of a conventional variableoptical attenuator using an actuator driven by an electrostatic force. Avariable, optical attenuator shown in FIG. 1 includes a substrate 11with a pair of optical fibers having a transmitting end 20 and areceiving end 30, respectively, an electrostatic actuator comprised ofdriving electrodes 12 a, 12 b, a ground electrode 14, a, spring 15 and amovable mass 16, and an optical shutter 17 connected to the movable mass16 of the electrostatic actuator.

[0009] The driving electrodes 12 a, 12 b and the ground electrode 14 areformed over the substrate 11 and supported by an oxide layer called an“anchor”. The movable mass 16 is connected to the ground electrode 14via the spring 15 and has a comb shape. The driving electrodes 12 a, 12b have respective extended portions 13 a, 13 b, each with a comb shape.The comb of each of the extended portions 13 a, 13 b is interdigitatedwith the comb of the movable mass 16.

[0010] When driving signals are applied to the driving electrodes 12 a,12 b so as to generate a potential difference between the drivingelectrodes 12 a, 12 b and the ground electrode 14, electrostatic forcearises between the interdigitated combs of movable mass 16 and extendedportions 13 a, 13 b, thereby driving the movable mass 16 to move. As themovable mass 16 moves, the optical shutter 17 is inserted into a gapdefined by the transmitting end 20 and the receiving end 30 so as topartially intercept beams of light incident onto the optical shutter 17.

[0011] It is important for the variable optical attenuator to vary anamount of attenuation based on wavelengths of incident light beams.

[0012] Further, it is important for the variable optical attenuator tominimize variation of a power level of the attenuated light beams, suchvariation being caused by a disturbance such as time passing,wavelengths of the incident light beams, polarization change of theincident light beams and vibration.

[0013] However, a conventional variable optical attenuator isdisadvantageous in that wavelength dependent loss (WDL) and polarizationdependant loss (PDL) are great because the optical shutter has a flatpanel shape.

[0014]FIGS. 2A and 2B illustrate schematic views of conventional opticalshutters in accordance with the conventional art.

[0015] Referring to FIG. 2A, light beams traveling from the transmittingend 20 of an optical fiber to a receiving end 30 of an optical fiber arepartially intercepted by an optical shutter 27. Here, the opticalshutter 27 is formed of the same silicon material as a known actuator.

[0016] Of the light beams incident to the optical shutter 27, a greatportion of light beams R is reflected by the optical shutter, so thatentry of the reflected light R to the receiving end 30 is prevented.However, since the optical shutter 27 is made of silicon having hightransmittance, a portion of the light beams T is allowed to be incidentto the receiving end, 30 of the optical fiber through the opticalshutter 27. Further, the light beams are scattered by reflection andtherefore scattered lights S1, S2 are generated. Of the scatteredlights, a portion S1 enters the receiving end 30 and the other portionS2 may be reflected back into the transmitting end 20. Accordingly, theconventional optical shutter 27 has a disadvantage of low light shutoffefficiency because the optical shutter 27 is made of silicon having hightransmittance. Therefore, to solve a problem of low light shutoffefficiency of the optical shutter 27, an optical shutter coated with areflective metal layer having high reflectivity (about 90%) is providedwith reference to FIG. 2B. The reflective metal layer is formed of amaterial of Au, Ni, Cu, Al and Pt.

[0017]FIG. 2B illustrates an optical shutter 37 coated with a reflectivemetal layer 37 a made of Au. The optical shutter 37 reflects almost oflight beams R incident onto the optical shutter 37, so that few of lightbeams may be incident onto the receiving end 30 of the optical fiber.

[0018] However, the optical shutter 37 coated with the Au layer 37 agenerates scattered lights S1, S2, and the scattered lights S1, S2 enterthe receiving end 30 and the transmitting end 20.

[0019] For example, in the case of attenuating light beams incident tothe optical shutter coated with the Au layer to 50%, 49% of light beamsR of the entire light beams passing out the transmitting end 20 may beintercepted by reflection from the Au layer on the optical shutter 37,but 1% of light beams are scattered and the scattered lights S1, S2enter the receiving end 30 and reenter the transmitting end 20.

[0020] However, even though the scattered lights S1, S2 are few, reentryof the scattered lights S2 to the transmitting end 20 by back reflectionincreases. Further, the amount of the scattered lights S1, S2 is subtlychanged based on wavelengths and polarization of the incident lightbeams. In the case that the scattered lights enter the receiving, end30, WDL and PDL increase, thereby deteriorating attenuation reliability.

[0021] As described above, the conventional MEMS variable opticalattenuator has a disadvantage of low reliability because an the amountof back reflected lights increases due to the low light shutoffefficiency of the optical shutter and because the WDL and PDL increase.

SUMMARY OF THE INVENTION

[0022] Therefore, the present invention has been made in view of theabove problems, and it is an object of the present invention to providean MEMS variable optical attenuator provided with an optical shuttercoated with a surface layer which has characteristics of highreflectivity and transmittance, thereby preventing light beams fromscattering and reducing the amount of back reflected light beams.

[0023] It is another object of the present invention to provide an MEMSvariable optical attenuator provided with an optical shutter having ashape capable of refracting light beams transmitted into the opticalshutter for preventing entry of the transmitted light beams to areceiving end of an optical fiber.

[0024] In accordance with one aspect of the present invention, the aboveand other objects can be accomplished by the provision of an MEMSvariable optical attenuator comprising a substrate having a planarsurface, a micro-electric actuator arranged on the planar surface of thesubstrate, a pair of optical waveguides having a receiving end and atransmitting end, respectively, and coaxially aligned with the otherwhile being arranged on the planar surface, an optical shutter movableto a predetermined position between the receiving end and thetransmitting end of the optical waveguides, and driven to move by themicro-electric actuator, and a surface layer formed on the opticalshutter, having reflectivity less than 80% so as for incident lightbeams to partially transmit thereinto, and having an extinction ratiowhich is varied based on a thickness thereof, thereby extinguishing thepartially transmitted light beams therein.

[0025] Preferably, the surface layer is formed of a material selectedfrom the group comprising Ti, TiO₂, Cr, CrO₂, W, Te and Be.

[0026] Preferably, the surface layer is formed of a double layercomprising a first layer formed of a material selected from the groupincluding Ti, Cr, W, Te and Be and a second layer formed of TiO₂ orCrO₂.

[0027] Preferably, the optical shutter has a flat panel shape and isarranged to be oblique relative to the transmitting end and thereceiving end of optical fibers.

[0028] Preferably, the optical shutter has a first surface perpendicularto an optical axis of the receiving end of the optical waveguide and asecond surface which is oblique with respect to the transmitting end ofthe optical waveguide with an inclination angle less than 90°.

[0029] Preferably, the optical shutter has a half wedge shape.

[0030] Preferably, the actuator includes an electrode section comprisinga ground electrode fixed onto the substrate and driving electrodes, aspring arranged on the substrate and connected to the ground electrodeat one end thereof, and a movable mass connected to the other end of thespring and arranged on the substrate to be movable toward the drivingelectrodes.

[0031] Preferably, the surface layer is formed of a material selectedfrom the group comprising Ti, Cr, W, Te and Be, and the electrodes arecoated with the same material as the surface layer.

[0032] In accordance with another aspect of the present invention, thereis provided with an MEMS variable optical attenuator comprising asubstrate having a planar upper surface, a micro-electric actuatorarranged on the planar upper surface of the substrate, opticalwaveguides having a receiving and a transmitting end, respectively, andarranged on the upper surface of the substrate to be coaxially alignedwith the other, and an optical shutter movable to a predeterminedposition between the receiving end and the transmitting end of theoptical waveguides, wherein the optical shutter has a first surfaceperpendicular to an optical axis of the receiving end and a secondsurface being oblique with respect to the transmitting end of theoptical waveguide with an inclination angle less than 90°.

[0033] Preferably, the optical shutter has a half wedge shape.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above and other objects, features and other advantages of thepresent.,invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

[0035]FIG. 1 illustrates a schematic perspective view of an opticalattenuator in accordance with the conventional art;

[0036]FIGS. 2A and 2B illustrate schematic views of flat panel shape ofoptical shutters used in conventional MEMS variable optical attenuators;

[0037]FIG. 3 illustrates a schematic view of an optical shutter used inan MEMS variable optical attenuator in accordance with one embodiment ofthe present invention, wherein the optical shutter is coated with a Tilayer;

[0038]FIG. 4 illustrates a schematic view of a half wedge-shaped opticalshutter of an MEMS variable optical attenuator in accordance withanother embodiment of the present invention;

[0039]FIG. 5 illustrates a schematic view of an optical shutter of anMEMS variable optical attenuator in accordance with further anotherembodiment of the present invention;

[0040]FIG. 6 illustrates a schematic perspective view of an MEMSvariable optical shutter in accordance with the present invention;

[0041]FIG. 7A illustrates a graph comparing wavelength dependence loss(WDL) of a conventional variable optical attenuator with WDL of avariable optical attenuator of the present invention; and

[0042]FIG. 7B illustrates a graph comparing a polarization dependenceloss (PDL) of a conventional variable optical attenuator with PDL of avariable optical attenuator of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] A detailed description of an MEMS variable optical attenuator inaccordance with preferred embodiments of the present invention will begiven below with reference to the accompanying drawings.

[0044]FIG. 3 illustrates a schematic view of an optical shutter used inan MEMS variable optical attenuator in accordance with one embodiment ofthe present invention.

[0045] Referring to FIG. 3, the optical shutter 47 has a flat panelshape and is coated with a Ti layer 48. Light beams traveling from atransmitting end 20 of an optical fiber to a receiving end 30 of anoptical fiber are partially intercepted by the flat panel shape ofoptical shutter 47.

[0046] The Ti layer 48 formed on the surface of the optical shutter 47is a light permeable material, unlike Au which has a reflectivity ofabout 98% and is used as a coating layer in a conventional opticalshutter. As compared with the reflectivity of Au, the Ti layer 48 has areflectivity of about 60%. That is, a light transmittance of the Tilayer 48 is about 40%. The Ti layer 48 has a transmittance lower thanthat of silicon serving as a body of the optical shutter but has acharacteristic of light extinction.

[0047] Accordingly, as shown in FIG. 3, when light beams are incidentonto the optical shutter 47 coated with the Ti layer 48, a portion ofthe beams R is reflected by the Ti layer 48 formed on the opticalshutter 47 so that the reflected lights R may not be incident onto thereceiving end 30 of the optical fiber. Further, a portion of the lightbeams transmits into the Ti layer 48 and is extinct in the Ti layer 48.Accordingly, the transmitted lights into the Ti layer 48 may not beincident onto the receiving end 30 of the optical fiber as well.

[0048] Further, the Ti layer 48 has low reflectivity, unlike the Aulayer used in the conventional optical shutter, so that light beamsscattered by reflection are reduced in number.

[0049] Accordingly, by using the optical shutter coated with the Tilayer, reliability of the variable optical attenuator is improvedbecause the wavelength dependence loss (WDL) and polarization dependenceloss (PDL) proportional to the amount of the scattered lights decreasedue to the reduced scattered light.

[0050] Since the optical shutter of this embodiment of the presentinvention is formed of silicon and Ti, both with high transmittance,light beams tend to be transmitted through the optical shutter.Accordingly, it is necessary to form the Ti layer to have a thicknesscapable of preventing the light beams from being transmitted through theoptical shutter.

[0051] Assuming that intensity of entire light beams passing out thetransmitting end 20 of the optical fiber is 100%, when the opticalshutter 47 shown in. FIG. 3 moves to a predetermined position betweenthe transmitting end 20 and the receiving end 30 for partiallyintercepting the light beams, 30% of the light beams are reflected froma surface of the Ti layer 48 formed on the optical shutter 47, and 20%of the light beams are transmitted into the Ti layer 48 and extinguishedtherein. Accordingly, the total amount of attenuation is 50%.

[0052] In the case of using the conventional optical shutter with Aucoating layer, 49% of the entire incident light beams are reflected bythe optical shutter. That is, 98% of the amount of attenuation isaccomplished by reflection from the shutter.

[0053] However, in the case of using the optical shutter with the Tilayer 48 in accordance with the embodiment of the present invention,only 30% of the entire incident light beams are reflected. Accordingly,the amount of the scattered lights originating from the reflected lightbeams is reduced. Further, since the transmitted light beams areextinguished in the Ti layer 48, the scattered lights originating fromthe transmitted light beams may be extinguished as well. Therefore, thescattered lights caused by the reflection of the incident light beamsand the back reflection of the transmitted light beams are reduced.

[0054] Consequently, since the amount of the scattered lights decreases,the WDL and PDL proportional to the amount of the scattered lightsgreatly decrease.

[0055] Instead of the Ti layer formed on the optical shutter forimproving light shutoff efficiency of the optical shutter, a metal layerformed of a material selected from a group including Cr, W, Te and Bemay be used. Further, TiO₂ or CrO₂ may be used for replacing the Tilayer because it has characteristics of light permeable and extinction.Further, the Ti layer may be replaced with a double layer comprised of afirst layer formed of a metal selected from a group comprising Ti, Cr,W, Te and Be and a second layer of TiO₂ or CrO₂. The second layer is asurface layer formed on the first layer.

[0056] In table 1, desired materials for coating the optical shutter areshown. Ti, Cr, W and Te used as the coating layer of the optical shutterhave a reflectivity less than 80%, thereby allowing light beams topartially transmit thereinto. The transmitted light beams areextinguished in the coating layer formed of Ti, Cr, W or Te because thecoating layer has a light extinction characteristic. Table 1 shows theextinction ratio of light beams based on thickness of the coating layer.Metals shown in table 1 are able to lower a reflectivity of the opticalshutter and extinguish the transmitted light therein by being coated onthe optical shutter, thereby minimizing influence of the scattered lightbeams on the shutoff efficiency of the optical shutter and improving ashutoff characteristic of the optical shutter. In table 1, physicalquantities are obtained assuming wavelength of the light beams is 1.5 μmwhich is a wavelength of light beams typically used in opticaltelecommunication. TABLE 1 Refractive Refractive index of Extinctionindex of imaginary ratio Material real number number Reflectivity(dB/nm) Ti 4.04 3.82 0.596 0.139 Cr 4.13 5.03 0.680 0.183 W 2.36 4.610.710 0.168 Te 7.23 0.48 0.574 0.107

[0057] Further, since natural oxides or oxides of the materials in table1 have the same characteristic as the materials in table 1, the naturaloxides and oxides of the materials in table 1 may be used as the coatinglayer formed on the optical shutter instead of the Ti layer 48.Accordingly, TiO2 and. CrO2 may be used as the coating layer solely ortogether with the metals shown in table 1.

[0058]FIG. 4 is a schematic view of a half wedge-shaped optical shutterof a variable optical attenuator in accordance with another embodimentof the present invention.

[0059] In this embodiment, an optical shutter 57 has the same Au coatinglayer as the conventional optical shutter but has a different shape fromthe conventional optical shutter. Unlike the conventional opticalshutter having a flat panel shape, the optical shutter in accordancewith this embodiment of the present invention has a half wedge shape sothat generation of the scattered light beams can be prevented and thelight beams transmitted into the optical shutter cannot be incident ontothe core region of the receiving end 30 of the optical fiber because thetransmitted light beams are refracted at surfaces of the half wedgeshaped optical shutter.

[0060] As shown in FIG. 4, the optical shutter 57 has a first surfaceperpendicular to an optical axis X1 of the receiving end 30 of theoptical fiber and has a second surface oblique relative to thetransmitting end 20 of the optical fiber.

[0061] An angle between the second surface of the optical shutter 57 andan optical axis X2 of the transmitting end 20 of the optical fiber arearbitrarily determined in the range of 0 to less than 90° as long as thelight beams transmitted into the optical shutter 57 can be refracted atthe second surface not to enter the core region of the receiving end 30of the optical fiber.

[0062]FIG. 4 shows dispersion of light beams traveling from thetransmitting end 20 of the optical fiber to the receiving end 30 of theoptical fiber, wherein the light beams are partially intercepted by thehalf wedge-shaped optical shutter. The optical shutter is formed ofsilicon, the same as other actuators.

[0063] About 60% of light beams can transmit the silicon materialserving as the body of the optical shutter. Accordingly, in theconventional optical attenuator, the optical shutter is coated with theAu layer having high reflectivity for preventing incident light beamsfrom penetrating into the optical shutter and entering the receiving endof the optical fiber through the optical shutter. However, suchconventional optical shutter with the Au coating layer causes a problemof deteriorating reliability of the variable optical attenuator byincreasing the WDL and EPDL which are proportional to the amount ofscattered light beams originating from the reflected lights and the backreflected light.

[0064] Accordingly, the present invention suggests the optical shutterwithout the reflective coating layer that generates scattered lights.The optical shutter of the present invention has a different shape fromthe conventional optical shutter, thereby preventing light beams fromentering the receiving end 30 of the optical fiber by reflecting aportion of the incident light beams and refracting the light beamstransmitted into the optical shutter.

[0065] Referring to FIG. 4, when light beams traveling from thetransmitting end 20 of the optical fiber to the receiving end 30 of theoptical fiber are incident onto the optical shutter 57, a portion of thelight beams are reflected by the optical shutter 57, thereby beingintercepted. Another portion of the light beams T is transmitted intothe optical shutter 57, refracted at the second surface (or a slantedsurface) of the optical shutter 57 and then refracted again at the firstsurface perpendicular to the optical axis of the receiving end 30 of theoptical fiber, thereby being diverted from the core region of thereceiving end of 30 the optical fiber 30. The transmitted light beams Tare refracted at the second surface by an angle which is the same as theangle between the slanted surface of the optical shutter 57 and a lightincident direction.

[0066] Accordingly, the half wedge-shaped optical shutter 57 shown inFIG. 4 does not generate scattered lights caused by reflection but onlygenerates a small amount of scattered lights S1, S2 by surfaceroughness. Therefore, the WDL and PDL are greatly decreased.

[0067] Further, it is preferable to combine advantages of the opticalshutter shown in FIG. 3 and advantages of the optical shutter shown inFIG. 4, thereby providing an optical shutter shown in FIG. 5.

[0068]FIG. 5 illustrates a half wedge-shaped optical shutter coated witha Ti layer. Referring to FIG. 5, when light beams traveling from atransmitting end 20 of an optical fiber to a receiving end 30 of anoptical fiber are incident onto an optical shutter 67 with a Ti layer68, a great proportion of the light beams R are intercepted byreflection in a similar way of the optical shutter shown in FIG. 4.

[0069] However, the optical shutter in accordance with the embodimentshown in FIG. 5 is different from the optical shutter in accordance withthe embodiment of FIG. 4 in that the scattered light beams and the lightbeams transmitted into the optical shutter 67 are extinguished in the Ticoating layer 68. Accordingly, entry of the light beams into thereceiving end 30 of the optical fiber is substantially prevented.Further, surface roughness of the optical shutter 67 is improved becausethe Ti layer 68 is coated on the optical shutter 67, so that thescattered lights are scarcely generated.

[0070] As described above, since the optical shutter 67 does not use anAu layer having a high reflectivity as a surface coating layer, amountof the scattered lights is greatly reduced. Accordingly, the WDL and PDLcaused originating from the scattered lights decrease, and consequentlyreliability of the variable optical attenuator is improved.

[0071] Further, in this embodiment of FIG. 5, it is not, necessary torigorously limit a thickness of the Ti coating layer. That is, in theembodiment with reference to FIG. 3, it is necessary to eliminate allthe light beams transmitted into the optical shutter by the Ti coatinglayer 48. Accordingly; the Ti coating layer 48 has to have a sufficientthickness for all the transmitted light beams to be extinguishedtherein. However, in the embodiment with reference to FIG. 5, since theoptical shutter 67 has a wedge shape, the light beams transmitted intothe optical shutter 67 are not directed to enter the receiving end 30 ofthe optical fiber by being refracted from the surfaces of the opticalshutter 67. Accordingly, it is not necessary to rigorously limit athickness of the Ti coating layer.

[0072] Accordingly, in the case of using the optical shutter shown inFIG. 5, since the amount of the scattered lights caused by reflection isreduced to a third of that caused by using the conventional Au coatedoptical shutter, and the transmitted lights are refracted at thesurfaces of the optical shutter, entry of the scattered lights to thereceiving end of the optical fiber is prevented.

[0073]FIG. 6 illustrates a schematic perspective view of an MEMSvariable optical attenuator provided with an optical shutter inaccordance with the embodiments of the present invention.

[0074] Referring to FIG. 6, an MEMS variable optical attenuator inaccordance with the present invention includes a substrate 71 havingoptical waveguides, preferably optical fibers, having a transmitting end20 and a receiving end 30, respectively, an electrostatic actuatorcomprising driving electrodes 72 a, 72 b, a ground electrode 74, aspring 75 and a movable mass 76, and an optical shutter 77 connected tothe movable mass 76. The driving electrodes 72 a, 72 b and the groundelectrode 74 are formed over the substrate 71 and supported by an oxidelayer 79. The movable mass 76 is connected to the ground electrode 74via the spring 75 at one end and suspended over the substrate 71. Thespring 75 has an elastic structure and is formed of a material the sameas the conventional actuators. The spring 75 has a curled structure withreference to FIG. 6, but is not limited thereto. The driving electrodes72 a, 72 b have respective extended portions 73 a, 73 b, each with acomb shape. The comb of each of the extended portions 73 a, 73 b isinterdigitated with the comb of the movable mass 76.

[0075] The optical shutter 77 has a half wedge shape with a slantedsurface facing the receiving end of the optical waveguide. Further, asurface layer is preferably coated on the half wedge shaped opticalshutter for improving a light shutoff efficiency of the optical shutter.The surface layer includes a Ti layer. Further, the electrodes, thedriving electrodes and the ground electrodes, may be coated with a layerformed of the same material as the surface layer on the optical shutter.If the surface layer for coating the optical shutter and the layer forcoating the electrodes are formed of the same material, the layer andthe surface layer may be simultaneously formed during the same processstep.

[0076] The optical shutter for use in the MEMS variable opticalattenuator in accordance with the present invention is capable ofminimizing influence of scattered light beams, thereby reducing the WDLand PDL proportional to the amount of the scattered lights and improvingreliability of the optical shutter.

[0077]FIG. 7A illustrates a graph comparing the WDL of the conventionaloptical shutter coated with a Au layer with that of the optical shuttercoated with a Ti layer in accordance with the present invention. The WDLis measured in the cases where attenuation amounts are 0 dB, 10 dB and20 dB, respectively. In the graph of FIG. 7A, the longitudinal axisindicates the amount of the WDL and bars indicate variation of the WDLamount.

[0078] Referring to FIG. 7A, in the case that attenuation amount is 0dB, almost no variation of the WDL is caused by the optical shuttercoated with the Ti layer, but the WDL ranging from 0.1 to 0.2 dB ismeasured in the optical shutter coated with the Au layer.

[0079] Further, in the case that attenuation amount is 10 dB, the WDLranges from 0.1 to 0.2 dB in the conventional optical shutter coatedwith the Au layer, but is only 0.1 dB in the optical shutter of thepresent invention.

[0080] Still further, in the case that attenuation amount is 20 dB, theWDL ranges from 0.4 to 1 dB in the conventional optical shutter but isonly 0.3 dB in the optical shutter of the present invention.

[0081] On the other hand, the optical shutter of the present inventionis superior to the conventional optical shutter in a characteristic ofthe PDL too. Referring to FIG. 7B, in the case that the attenuationamount is 0 dB, the PDL scarcely varies in the optical shutter of thepresent invention, which has a half wedge shape and Ti coating layer,and the convention optical shutter coated with the Au layer. However, inthe case that the attenuation amount is 10 dB, the PDL ranges from 0.1to 1 dB in the conventional optical shutter, but is only 0.2 dB in theoptical shutter of the present invention. Further, in the case of theattenuation amount of 20 dB, the PDL ranges from 0.7 to 1.6 dB in theconventional optical shutter, but is only 0.2 dB in the optical shutterof the present invention.

[0082] As described above, the variable optical attenuator with theoptical shutter in accordance with the embodiments of the presentinvention is capable of minimizing variation of the light intensity ofthe attenuated light beams by suppressing generation of the scatteredlights.

[0083] Although the preferred embodiments of the present invention havebeen disclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. An MEMS variable optical attenuator comprising: asubstrate having a planar surface; a micro-electric actuator arranged onthe planar surface of the substrate; a pair of optical waveguides havinga receiving end and a transmitting end, respectively, and coaxiallyarranged on the planar surface; an optical shutter movable to apredetermined position between the receiving end and the transmittingend of the optical waveguides, and driven to move by the micro-electroactuator; and a surface layer formed on the optical shutter, havingreflectivity less than 80% so as for incident light beams to partiallytransmit thereinto, and having a light extinction ratio to a thicknessthereof, thereby extinguishing the partially transmitted light beamstherein.
 2. The MEMS variable optical attenuator as set forth in claim1, wherein the surface layer is formed of a material selected from agroup comprising Ti, TiO₂, Cr, CrO₂, W, Te and Be.
 3. The MEMS variableoptical attenuator as set forth in claim 1, wherein the surface layer isformed of a double layer comprising a first layer formed of a materialselected from a group including Ti, Cr, W, Te and Be and a second layerformed of TiO₂ or CrO₂.
 4. The MEMS variable optical attenuator as setforth in claim 1, wherein the optical shutter is a flat panel shape andarranged to be oblique between the transmitting end and the receivingend.
 5. The MEMS variable optical attenuator as set forth in claim 1,wherein the optical shutter has a first surface perpendicular to anoptical axis of the receiving end of the optical waveguide and a secondsurface oblique relative to the transmitting end of the opticalwaveguide with an inclination angle less than 90°.
 6. The MEMS variableoptical attenuator as set forth in claim 4, wherein the optical shutterhas a half wedge shape.
 7. The MEMS variable optical attenuator as setforth in claim 1, wherein the actuator includes: an electrode sectioncomprising a ground electrode fixed onto the substrate and drivingelectrodes; a spring arranged on the substrate and connected to theground electrode at one end thereof; and a movable mass connected to theother end of the spring and arranged on the substrate to be movabletoward the driving electrodes.
 8. The MEMS variable optical attenuatoras set forth in claim 7, wherein the surface layer is formed of amaterial selected from the group comprising Ti, Cr, W, Te and Be, andthe electrodes are coated with the same material as the surface layer.9. An MEMS variable optical attenuator comprising: a substrate having aplanar upper surface; a micro-electro actuator arranged on the planarupper surface of the substrate; optical waveguides having a receivingend and a transmitting end, respectively, and coaxially arranged on theupper surface; and an optical shutter movable to a predeterminedposition between the receiving end and the transmitting end of theoptical waveguides, wherein the optical shutter has a first surfaceperpendicular to an optical axis of the receiving end and a secondsurface oblique relative to the transmitting end of the opticalwaveguide with an inclination degree less than 90°.
 10. The MEMSvariable optical attenuator according to claim 9, wherein the opticalshutter has a half wedge shape.